Removing ridges from edges of thick acrylic

Community Support
1

I usually cut wood with my GF, but recently got a new project that uses 1/4" black acrylic. I’m having issues with the cleanliness of the cut sides, they have ridges that I can’t eliminate. Other posts on this forum indicate this is a natural thing, but the rough, ridged edges on acrylic just aren’t professional enough for me to sell as a product. I’m looking for ways to reduce or eliminate these ridging artifacts.

Here’s a 1"x1"x.22" test cut where you can see the vertical ridges. This is what I need to eliminate.

plastic_with_ridges
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What I’ve tried:

  • Using black masked cast acrylic .22" thick from Lesnlok
  • I tried other acrylic brands but it was inconsistent and lasered poorly (melted without cutting through, etc)
  • The best setting is (unsurprisingly) the Proofgrade thick black acrylic setting (116/full)
  • Cleaned the GF thoroughly, including lenses
  • Tried to defocus the laser, no difference (put in a different material height)
  • Adjusted belt tension and ensured v-wheels are smooth and running nicely on the rails
  • Tried different settings for speed and power, including running multiple passes vs 1 pass, etc
  • At slower speeds, the charring is pronounced and the edges start to crumble
  • I’ve tried sanding the edges, but the shape is curved and irregular, and precision is important so I can’t over- or under-sand. Plus, it’s difficult to get into some of the curves. Finally, at production scale, this would increase production time significantly.
  • Multiple passes seem to burnish the edges, but the corners become crumbled (see below)

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  • I’m flame-polishing the edges with a kitchen torch, to soften the corners, but that only mellows the ridges without eliminating them:

flamepolish_sidebyside
flamepolish_sidebyside1128×1000 246 KB

Additional research found a very interesting video that shows the effect of different speeds and exhaust blowing pressure on how the edges turn out: https://www.youtube.com/watch?v=1U2AYkQ1DLc
It seems that reducing the blowing pressure on a slower speed helps reduce the edge artifacts. However, Glowforge doesn’t provide software tools to adjust the fan speed (which I’m sure has some good reasons, but still). There is a hardware hack I saw to place an air baffle above the fan, limiting the amount of air being blown at the head, but that seems extreme and I’m not interesting in sparking a flaming ball o’ laser.

Has anyone got any suggestions I haven’t tried yet? I need to get this issue solved soon, the launch date is a few weeks out and I need to make sure I’ve got enough stock to fulfill orders.

Thanks for any help pointing me in the right direction.

2

I have had good luck with high-grit wet sanding (including a sequence of Zona papers) followed by polishing compound, but it is a lot of work and can be challenging for curves.

if you can get one piece exactly the way you want it, could you make a silicone mold, then cast the pieces in acrylic resin? That will generally give you a consistent finish similar to your original.

3

Based on your settings, you have a 40W Glowforge Basic/Plus, which must have a very old tube. It’s really difficult to cut 1/4" acrylic cleanly with only that amount of power. Cutting with two passes gives me smooth edges but they are not visibly clear any more, so you’re back to some combination of flame polishing and wet sanding. The right answer may be to have someone else cut these in bulk using a more powerful laser with better air assist, or to buy a more powerful laser with better air assist.

4

Or try some blue tape over the air nozzle with some holes poked into it. Might be able to get enough air volume to keep the lens clean without creating a wavering flame path that results in striations & ridges.

5

Thank you for the suggestions and insights. It doesn’t sound like I’ve missed anything obvious, and further testing with speeds ‘n feeds have not improved the random reeding artifacts.

The casting my own resin idea is an intriguing one, with its own challenges (I do other smaller-scale resin casting), but I’m not sure that would scale for this project. The wet sanding idea is a good one also, although having to hand-sand hundreds of 6x6” irregular-shaped objects gives me nightmares. I might just do it for the top side, which is mostly straight.

Thanks!